New Coastal Flooding Products Help Us Track Hermine’s Storm Surge

Posted on September 1, 2016 By

Hurricane Hermine Usurge
Note:  This blog post originally appeared on Hurricane Hal’s Storm Surge Blog and is reblogged with permission from the the author.
Tropical Storm Hermine is now a strong tropical storm, with maximum sustained winds of 65 mph, as of the National Hurricane Center advisory at 7:00AM Central time. Hermine is forecast to strengthen a bit more, possibly becoming a hurricane before it makes landfall near Florida’s Big Bend/ Apalachee Bay.

 

Tropical Storm Hermine is forecast to strengthen as it approaches landfall near Florida’s Big Bend/ Apalachee Bay. 

New coastal flooding products are helping us track Hermine’s storm surge.  The National Hurricane Center’s new Potential Storm Surge Flooding Map provides a map of water levels that have around a 10% chance of being exceeded. The map below shows that areas of Jefferson and Taylor counties, along the shore of Apalachee Bay, have around a 10% chance of observing 6 ft (1.83 m) or more of water above ground level.
The National Hurricane Center has released a new storm surge inundation product, which provides users with a map of potential water levels above ground

 

This tool has brought storm surge forecasting into the realm of geospatial analysis, using the SLOSH modeling expertise of the National Hurricane Center’s Storm Surge Unit. This product will be very helpful for decision making as tropical weather systems approach coastlines.
The U-Surge project is also providing historic storm surge analog maps, which enable us to see a map of coastal flooding with previous storms that took similar tracks. This map of storm surge from Tropical Storm Alberto (2006) is available at Tropical Storm Hermine’s U-Surge page.
Tropical Storm Alberto tracked into Apalchee Bay as a strong tropical storm with maximum sustained winds 70 mph (60 knots) in 2006. Alberto generated storm tide levels exceeding 8 ft (1.22 m) in Dixie County, and 2-4 ft in the Tampa area.

 

A map of Tropical Storm Alberto’s storm track and maximum coastal flooding levels.
 
With such analogs, it is important not to focus on the exact track or to consider the analog a forecast for the present storm. However, analogs provide us with insights into storm surge patterns that are evident with certain storm tracks.
For example, Alberto’s map shows a clear distinction between high water levels to the east of the storm track, and relatively low water levels to the west. Although the storm made landfall more than 150 miles north of Tampa, it still generated a 2-4 foot storm tide in that area. We should expect coastal flooding in these areas with Hermine, as well.
Although several feet of storm surge may not sound threatening for places like Tampa, St. Petersburg and Clearwater, flood impacts are often exacerbated in metro areas as the storm surge impedes the drainage of heavy rain that falls on hard surfaces. Such flooding was evident during Tropical Storm Debby in 2012.
A combination of heavy rain and prolonged storm surge led to flooding in the Tampa metro area during Tropical Storm Debby in 2012. Photo: Jason Behnken
 
The combined impact of storm surge and heavy rain is a major concern for coastal cities. Thomas Wahl was the lead author on a paper published last year in Nature Climate Change that found an increasing trend in compound rain/ surge events for major U.S. cities.
Such compound flood events will become more severe over time even if storm frequency does not increase, due to rising seas associated with climate change.
Historical inundation graphs are another useful tool if you’re interested to track Hermine’s water levels relative to past coastal flood events. Hermine’s U-Surge page provides historical graphs for Cedar Key and Apalachicola in this new product that has just been launched.
Track Tropical Storm Hermine’s water level in the context of historical flood events and sea level rise at Hermine’s U-Surge page.
 
Highest total water levels are likely to occur during late afternoon/ evening in Apalachicola. High tide occurs after 4PM, so total water levels may reach maximums between 4-6PM, as the storm is still approaching at the time of high tide.
Cedar Key has a low tide at 9PM, so expect water levels to remain nearly stationary through the evening, instead of dropping, like normal. Then expect water levels to rise rapidly after 9PM, as increasing tidal levels and storm surge combine to elevate total water level. Maximum water levels should occur after midnight, as strong westerly winds behind the back of Hermine continue to push storm surge as the 3AM high tide approaches.
In locations farther removed from Hermine’s track, like Tampa, water levels will not rise so suddenly. Expect a gradual water level rise in the metro area, with flooding most likely associated with heavy rain bands that have trouble draining into the elevated coastal waters.

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The Maps are not the Problem: In Perpetuating a Myth, Horowitz Misses the Key Point

Posted on June 13, 2016 By

NOLA Map

 

Andy Horowitz’s recent New York Times opinion piece on the new flood insurance rate maps for New Orleans contains a number of inaccurate statements, fails to address the fundamental issue, and amounts to nothing more than picking on the “cash-strapped New Orleanians” as they continue the long process of recovering from catastrophic levee failures that occurred over a decade ago. Many of the shortcomings of this piece have been pointed out here and here. In addition to the factual errors that these critiques raise, Prof. Horowitz perpetuates a fundamental misunderstanding of flood risk and how our nation manages it. In no way do the new maps suggest that flood risk has been eliminated, as Prof. Horowitz claims. In perpetuating this inaccurate cliche, the article dances around but does not directly address a fundamental flaw in the National Flood Insurance Program.

“Safe from Flooding” is a Fiction

Prof. Horowitz’s statement that the new flood maps “declared most of New Orleans safe from flooding” is incorrect. The new maps merely moved some areas from the 1% annual chance flood risk area, the “100-yr flood zone” or the “special flood hazard”, to areas that are estimated to have an 0.2% chance of flooding during any given year, the so-called “500-yr flood zone.”  (A flood that has a 1% annual chance will have an average return of 100 years, while a 0.2% annual chance corresponds to an average return period of 500 years.)  This adjustment is an accurate reflection of the risk reduction provided by the Federal, state, and local investment in an improved levee, floodwall, and pump station system. (However, FEMA’s methodology does not account for the flood risk reduction benefits provided by numerous coastal restoration and community resilience projects.)

The article wrongly perpetrates the misconception that properties in the 500-yr floodzone are not considered at risk. As Prof. Ed Link, a nationally recognized flood risk expert at the University of Maryland, put it: “there is no mention of safe within the language of the FEMA program or the maps.”  In repeating this myth, Prof. Horowitz completely misses the point that these properties are at risk, that the maps accurately depict this risk, and that these property owners should pay into the flood insurance risk pool.

 

Mirabeau Gardens

FEMA’s online flood hazard webmap for New Orleans shows that the area around the London Avenue Levee Exhibit Hall and Rain Garden, what Prof. Horowitz describes as “the area around a new memorial exhibition at the site of a catastrophic levee failure”, is not shown to be safe in the new maps as he claims. The maps very clearly show that the area is subject to the 0.2 percent annual chance flood hazard. (By the way, the maps do not account for the rain garden that is part of the exhibit, another one nearby, or the many other “green infrastructure” projects in the area that are known to reduce flood risk.)

 

NOLA Map

Zoomed out to the entire city, FEMA’s flood zone webmap for New Orleans shows that there are no areas in the city that have been declared safe, as Prof. Horowitz claims.  Likewise, for neighboring St. Bernard, Jefferson, and Plaquemines parishes.

 

New Orleans’ Flood Maps Advanced the State of the Practice

Prof. Horowitz is also incorrect to state that the flood risk analysis did not account for sea level rise, subsidence, or the probability of levee failures. All of these factors were considered in the technical analysis. It was a complex and tricky process, and there are legitimate questions about the assumptions and methods used. But what he bemoans as “six years of waiting and wrangling” was in fact a series of intensive consultations between national and local technical experts dealing with the very challenges that Prof. Horowitz claims were ignored.  This is what it takes develop, in an open and public manner, flood risk maps that adhere to national standards while also including local data and knowledge.  The result is likely the nation’s most comprehensive, computationally intensive, mathematically rigorous, and transparent attempt to estimate flood risk for a region. The northeast coast in the aftermath of Hurricane Sandy is the only region that we know of that comes close to the effort to create flood risk maps that accounts for changes in the climate and landscape.

In the face of these challenges, this is the effort it takes to have hydrologically correct flood risk maps for our coastal cities. Further, hydrologically correct flood risk maps is a necessary condition for an actuarially correct flood insurance program, the stated goal of many years of effort at flood insurance reform.

The implicit assumption throughout the article is that the maps are an inaccurate reflection of flood risk for the region, but the article does not attempt to make that case. In fact, his statement that the maps “reflect the most optimistic possible assessment” is a tacit acknowledgement that the flood risk depicted on the maps is in fact within the confidence interval of the technical analysis. It is clear the Prof. Horowitz does not like the outcome of the maps, and he does raise legitimate concerns regarding the outcome. However, in attacking the maps and the process that created the maps, he failed to address the fundamental problem with flood insurance in New Orleans or elsewhere in America.

Flood Insurance Risk Pool Needs to Include the 500-yr Floodzone

In moving areas from the 100-yr to 500-yr flood zone, the maps in no way imply that areas are safe. In fact, they explicitly state that there is a 0.2% chance that flooding will occur during any given year. The fundamental problem is that the flood insurance program currently exempts these properties in the 500-yr flood zone from flood insurance requirements. Because they are not required to pay into the risk pool, these residents are less likely to be aware of their flood risk. Expanding the risk pool to include these properties increases flood risk awareness of these residents. It is also another necessary condition for an actuarially correct and financially sustainable flood insurance program. The failure to address this fundamental flaw in the flood insurance program largely explains why attempts to reform the flood insurance program have failed. Arbitrarily changing the flood maps for New Orleans because Prof. Horowitz does not like the outcome does nothing to address this flaw.

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Severe Weather, March 2016: Rapid Event Summary

Posted on March 11, 2016 By

  • Flash flooding across most of Louisiana along with parts of Texas, Arkansas, Mississippi, and Tennessee.

  • I-10 at Louisiana-Texas state line reopens after 60 hour closure.  

  • 6 fatalities reported and 2 people missing 

  • Over 5,000 people rescued from flood waters including swift water rescues

  • Estimated 18,000 homes suffer some degree of flood damage, consisting of 11,000 in Louisiana, 5,000 in Texas, and 2,000 in Mississippi. 

  • DisasterMap.net Estimate:  $1.2 billion in damage to structures, vehicles, and due to closure of I-10.

This page was last updated on April 19.

 

Event Description:

A severe weather system driven by an “atmospheric river” created a prolonged rain event that caused deadly and destructive flashing flooding in Texas, Louisiana, Mississippi, Arkansas, and Tennessee.  Parts of Louisiana received as much as 20″ of rain.  Additionally, prolonged onshore winds caused minor coastal flooding in Texas and Louisiana.  6 possible storm related deaths so far and an estimated 6,000 people have been forced from their homes.

 

Summary of Hazards:

Heavy rainfall induced flash flooding and coastal flooding.

Observed cumulative (March 4 – 11) rainfall amounts include:

– Nearly 22″ observed near Monroe, La
– Over 12″ observed in Covington, La and points north
– A large region in central Louisiana that experience 10″ or greater the inlcudes Shreveport, Monroe, and Natchitoches
– A major 5″ contour line covering a large region that includes Houston, the outskirts of Dallas, Memphis, and Jackon, Ms
– A second 5″ contour line that includes Baton Rouge, Slidell, Hattisburg, and McComb.

 

The cumulative precipitation maps shown below were generated by DisasterMap.net using data from the National Weather Service.  The 7-day cumulative precipitation map (CONUS) was downloaded for the March 4 – 11, 2016 period as a point shapefile.  From the point shapefile, a raster interpolation was completed and then contour lines were generated using the raster.  The processing and mapping was completed using QGIS.

The first map below shows the generated contours lines and the original NWS point dataset (at 50% transparency.)  The second map below shows the 5″ and 10″ contour lines with the original point dataset (filtered to only show values over 2″ and at 50% transparency) over a OpenStreetMap basemap (served by MapQuest.)

CumulativePrecip_Mar4-112016

 

CumulativePrecip_Mar4-112016_5&10inContour

 

 

 

Wind driven high tides extended from southern tip of Texas to the middle of the Florida panhandle.  3-4 ft tides measured in and between Galveston Bay and Bay St Louis.  Over 4 ft high tides were observed at two stations along the central Louisiana coastline.

TidalMap_NeedhamSource:  Hal Needham of Marine Weather & Climate and U-Surge.

DisasterMap.net’s Estimate of Impacted Populations:

This section presents the results of an initial attempt to estimate size of the population impacted by this event.  It is based on geoprocessing steps completed in a Geographic Information System (GIS).  It utilizes two hazards layers, rainfall contours and flooded areas, along with 2010 US Census population data.  It represents a rough estimate at a particularly moment in time.  The data contains uncertainties, and the methods while quick are imperfect.  In addition, this event continues to unfold.  The results are at best a rough estimate representing the particular moment in time captured by the hazard data.  

Using QGIS software, the 2010 US Census Demographic Profile 1 geodatabase was clipped by the 5″, 10″, and 12″ rainfall contours described above.  This procedure identified the Census tracts that experienced rainfall by these amounts or greater. For the clipped tracts it was then possible to sum the population and housing units.  Naturally, being within any of these contours does not necessarily imply direct flood impacts.  However, it does mean that indirect flood impacts, such power outages, isolation by waters, cut off from medical services, or work/employment impacts, are likely.  This is particularly true for the 12″ contour.

The table below summarizes the population and houses within each of Census tracts that overlapped the different contour lines, while the map shows these Census tracts color coded by population.

Within 12″ Contour Within 10″ Contour Within 5″ Contour
Total Population 453,773 891,344 8,974,962
Total Housing Units 203,600 404,204 3,774,001

CumulativePrecip_Mar4-112016_PopulationExposure

Applying a similar procedure using a flooded area polygon obtained from the Dartmouth Flood Observatory, it is possible to obtain an initial estimate of the population and homes located within Census tracts that experienced flooding.  For this early assessment, the “MSW_2016066_100W040N_3D3OT_V” flood zone shapefile was obtained from their repository, and then the Census DP1 layer was clipped by it.  The Dartmouth Flood Observatory provides continuous and global monitoring of floods using stream gauge and satellite data.  Their flooded areas shapefiles are produced through a land/water classification of primary MODIS satellite data, which has a 250 m spatial resolution.  Our steps identified Census tracts that overlapped with the flooded areas identified by Dartmouth.  Importantly, not everyone within a Census tract that overlaps the flooded areas actually flooded.  They may have an elevated home or the flood may be on the other end of their tract.  Still this provides a reliable early measure of the magnitude of the emergency response.

The table below summarizes the population and houses within each of Census tracts that overlapped the flooded areas identified by the Dartmouth Flood Observatory:

Within Census Tracts that Experienced Flooding
Total Population 2,166,501
Population Under 5 yrs Old 129,358
Population 85 yrs and Older 34,029
Total Housing Units 1,067,395
Occupied Housing Units 847,452
Owner-Occupied Housing Units 655,997
Renter-Occupied Housing Units 191,455

DartmouthFloodArea_PopExposure

Flood Data Source:

Citation: Brakenridge, G.R., Anderson, E., Kettner, A.J., Slayback, D. and Policelli, F., March 13, 2016, “Current Flood Conditions, MSW_2016066_100W040N_3D3OT_V “, Dartmouth Flood Observatory, University of Colorado, Boulder, CO, USA, http://floodobservatory.colorado.edu/.

DisasterMap.net’s Estimate of Economic Impacts:

DisasterMap.net estimates at least $1.2 billion in damage to homes, vehicles, and due to the closure of I-10.  This includes an estimated $900 million in flood damage to structures.

 

Flood Damage to Buildings and Contents

An estimated 18,000 structures have suffered flood damage. If the average cost of damage is $50,000 each, then the total direct damage to structures could reach $900 million. Additionally the contents of 18,000 homes also suffered flood damage. If they average $10,000 in contents damage per house, then that would total $180 million

Flood Damage to Vehicles

While flooded vehicles have been documented in media and social media, no estimate of flooded vehicles have been made available. In the absence of a ground based assessment, it is reasonable to assume that at least 6,000 vehicles flooded (1 for every 3 homes that flooded). If they averaged $10,000 in damage, then the total would be $60 million.

 

Cost of I-10 Closure

DisasterMap.net estimates that the I-10 closure will have an economic impact of the order $22.5 million, based on a 2.5 day closure and $9 million per day.

A Texas Department of Transportation and Development press release states that I-10 was closed on the morning of Wednesday, March 16.  At approximately 8 pm on March 18, the interstate reopened to traffic after the water receded and emergency repairs were made to the eastbound lane.  The interstate was closed for approximately 60 hrs.

(DisasterMap.net has previously assessed that “According the NWS river gauge nearest I-10, the Sabine river was at 7.36 ft at 9am on 3/16, approximately when the interstate flooded.  This gauge is forecast to remain at or above this height for approximately 55 hrs, or just over 2 days.”)

A study of two earlier interstate closures, due to flooding and an avalanche, found that the closures cost on average $9 million per day.  For a 4 day closure due to the flood, they found $47 million in economic damages,  $11.75 million per day.  For the 4 day closure due to the avalanche, they estimated $28 million, $7 per day.  Averaged, that gives $9 million per day.

 

Additional Damage not Estimated

In addition to the classes of damage described above, other types of known flood damage have not been tallied here. This includes crop losses, infrastructure (specifically a number of bridges, culverts, and roads are known to be damaged), and business and employment impacts. Additionally, emergency response and long term recovery costs have not been tallied.

 

Impact Reports:

 

Below is a summary of key impacts reported by the press and government agencies:

Major Disaster Declaration for Louisiana

Map of Declared Parishes

AP reports drowning deaths in Louisiana, 1 in Texas, and 1 in Oklahoma

AP reports at least four deaths in Louisiana

Weather Channel reports 6  flood related deaths

1 vehicular drowning in Bienville Parish, LA

2 fishermen missing from Claiborne County, MS

Evacuations in Southeast Texas as flood pulse moves down Sabine River 

12,500 people evacuated from flooded areas of Orange and Newton

LANG has rescued over 4,255 citizens and 354 pets

LANG rescues up to nearly 3,300 people and 316 pets

LDWF rescues 777 people

Louisiana National Guard rescues over 2,000 people

Louisiana Department of Wildlife and Fisheries rescues nearly 600 people

Louisiana tallies 11,000 flood damaged homes

Nearly 700 homes suffer major damage, another 4,500 damaged by floodwaters in Texas

Mississippi reports 1,600 affected by floodwaters and over 600 with major flood damage

Louisiana reports  6,143 structures have received flood damage across the state

Nearly 5,000 homes in Louisiana, 800 homes in Mississippi impacted by flooding

4,000 homes impacted by flooding in Bossier City, LA

Over 600 structure flooded in St. Tammany Parish, LA

Over 1,300 people rescued from flood waters in Louisiana

200 flood rescues in Hammond, LA

Nearly 1,000 homes flooded in Mississippi

Hundreds of homes flooded in Mississippi, 2 fishermen reported missing on the Mississippi River

40 shelters open at the peak

Over 100 people rescued in Bogalua, LA

Over 100 people rescued in and around Hammond, LA

50 people rescued from flood waters in Covington, LA

Approximately 50 homes evacuated in Chicou County, AK

Quachita River flowed backwards north of Monroe, LA

Rainfall causes sinkhole in Vicksburg, MS area

10 people rescued from flooded homes in Memphis, TN

Shelter population in Texas peaks 500 on March 15, 2016

Social Media from the Scene:

 

 

 

 

 

 

 

Stay Informed in Real time with DisasterMap.net:

 

Our homepage includes a Leaflet webmap with key hazard warning layers, and Full Size Map let’s you interact with the data to find out information specific to your location.  It also provides official warnings and guidance from the relevant government agencies through an RSS feed.  Our Twitter feed gives you reports, photos, and videos direct from the ground.  Follow us on social media to get updates.  We strive to bring you the sights and sounds of the impacted areas.

Capture03122016

 

Click the “View Full Size Map” to get to the fully interactive webmap, which include shelter locations.  Click one of the blue crosses to get the name, address, and capacity of any shelter that is part of FEMA’s National Shelter System.

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Use our address search to view conditions for areas where you are concerned.  And, our real time traffic layer displays any roads closed due to flooding.

 

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